This invention relates to a method and apparatus for providing a selectable reference voltage for a monolithic digital-to-analog converter (DAC). A reference current is typically generated by a voltage-to-current (V-to-I) converter from a reference voltage. In the prior art, the reference voltage is provided to the V-to-I converter of the DAC in one of two ways. It is either provided externally and connected to an input terminal of the DAC, or it is generated internally on the chip using an on-chip voltage reference generator.
Prior art DACs do not provide for the possibility of using either an internal on-chip reference voltage generator circuit or, in the alternative, an off-chip reference voltage source. The purpose of this invention is to provide a DAC which can accept, in the alternative, either an on-chip reference voltage or an externally generated reference voltage.